Method for fabrication of polycrystalline films

ABSTRACT

A mixture of two volatile solvent liquids and a particulate material is prepared, applied to a substrate and the liquids are evaporated from the mixture to form a film of the material on the substrate. One of the liquids has higher volatility but is a relatively poorer solvent for the material than the other liquid. Evaporation of the liquids may be followed by densification steps wherein a vaporized solvent is applied to the film and then evaporated.

United States Patent 1 Larrabee Oct. 23, 1973 METHOD FOR FABRICATION OFPOLYCRYSTALLINE FILMS [75] Inventor: Robert Dean Larrabee, Princeton,

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Oct. 28, 1971 [21] App]. No.: 193,541

[52] U.S. Cl 117/201, 117/63, 117/119.6, l17/1l9.8, 260/707 [51] Int.Cl. B44d 1/48, B44c 1/18 [58] Field of Search 117/201, 119.6, 119.8,117/63; 260/707 field, Chem. Abstracts, Volume 75, No. 134426(a), (Pg.329-330), 1971.

Kuapil, Growing Single Crystal Ferroelectric Compounds of Glycine, e.g.Triglycine Sulfate, Chem. Abstracts, No. 39566p (1968), pg. 3709, Vol.69. Varikash, Diffusion of Triglycine Sulfate in concentrated Aq. Sol.,Chem. Abstracts, Vol. 69, No. 99998r, pg. 9380, (1968).

Primary Examiner-Alfred L. Leavitt Assistant Examiner-M. F. EspositoAttorney-G. H. Bruestle [57] ABSTRACT A mixture of two volatile solventliquids and a particulate material is prepared, applied to a substrateand the liquids are evaporated from the mixture to form a film of thematerial on the substrate. One of the liquids has higher volatility butis a relatively poorer solvent for the material than the other liquid.Evaporation of the liquids may be followed by densification stepswherein a vaporized solvent is applied to the film and then evaporated.

17 Claims, No Drawings METHOD FOR FABRICATION OF POLYCRYSTALLINE FILMSBACKGROUND OF THE INVENTION This invention relates to a process fordepositing a film of material on a substrate and more particularly to aprocess utilizing two volatile solvent liquids to for a mixture with thematerial to be deposited.

In the development of infrared radiation detectors, studies have beenmade in search of various materials that may be used for such detectors.The object of one of these studies was to determine whether usefulpyroelectric materials such as triglycine sulfate (TGS), triglycineselenate (TGSe) and triglycine fiuoberyllate (TGFB) could be prepared inthin film form.

Thin films of many materials can be prepared by vacuum evaporation orsputtering techniques. However, TGS, TGSe and TGFB are delicate organiccompounds that thermally decompose before they melt or exhibitappreciable vapor pressure. For example, TGS melts with decomposition atabout 223C, whereas TGSe decomposes at about 150C. Therefore, since suchdecomposition precludes the use of evaporation techniques and makessputtering unattractive, there is a need for yet another method ofdepositing a pyroelectric material on a substrate.

In attempts to deposit pyroelectric films by solvent evaporation from asaturated aqueous solution, it was discovered that these pyroelectricmaterials formed relatively large (i.e., several millimeter) needle-likecrystals on an otherwise bare substrate. Because of the grossnon-uniformity of these layers, such a method of film deposition wasfound to be unsatisfactory.

Another method was suggested in U.S. Pat. No. 3,51 1,991 issued to H. P.Beerman. An essential step in this method, however, requires the use ofa plastic binder to hold microcrystals of the pyroelectricmaterialtogether. Unfortunately, the use of such binders or other additives mayhave an adverse effect on the properties of the resultant film andtherefore also is undesirable.

SUMMARY OF THE INVENTION In view of the disadvantages of the'foregoingmethods, a process has been found that produces a relatively densepolycrystalline film on a substrate without the use of any binders orother additives. Instead, the films are held together and bonded to thesubstrate by the DETAILED DESCRIPTION In a general embodiment of thepresent invention, two liquids are combined with a particulate materialto form a mixture. The liquids are so selected that one liquid hashigher volatility but is a poorer solvent for the particulate materialthan the other liquid. The major portion of the particulate material isheld in suspension while a lesser portion is dissolved in the liquids.After formation, the mixture is applied to a substrate and the liquidsare evaporated. Because one liquid has higher volatility than the other,this liquid evaporates at a faster rate than does the lower volatilityliquid. If the volatilities of the two liquids differ sufficiently, thehigher volatility liquid will completely evaporate while only a smallpercentage of the lower volatility liquid evaporates. During evaporationof the higher volatility liquid, the suspended particulate material inthe mixture gradually settles onto the substrate thereby building up afilm. Meanwhile, a portion of the material that was dissolved in thehigher volatility liquid remains in a solution and adds to the materialconcentration in the lower volatility liquid.

At the point of complete evaporation of the higher volatility liquid, itis highly desirable to have a sufficient amount of the lower volatilityliquid left to thoroughly saturate the deposited film so that the innergaps in the polycrystalline structure of the film contain the lowervolatility liquid having highly concentrated proportions of the materialdissolved therein. Subsequent evaporation of this lower volatilityliquid, therefore results in deposition of the dissolved material withinthe polycrystalline gaps, thereby increasing the density of thedeposited film.

By appropriate calculation or by experimentation, it is possible todetermine the proportions of the two liquids and the particulatematerial. Generally, it is desirable to hold the ratio of the lowervolatility liquid to particulate matter to within defined limits. Theideal value for this ratio occurs when the deposited film is adequatelysaturated with the lower volatility liquid after the higher volatilityliquid is evaporated. The upper limit of this same ratio is set to keepthe lower volatility liquid from completely dissolving the depositedfilm after the high volatility liquid has evaporated.

The ratio of particulate material to total liquid also can be varied andwill generally be determined by the procedure with which the mixture isapplied to the substrate. For example, the texture of the mixture can bevaried from a liquid to a thick paste to suit the method of applicationto the substrate and/or the thickness of film desired.

In order to increase the density of the resultant film, the generalembodiment may include one or more densification steps. Essentially,density of the film may be increased by dissolving portions on thesurface of the film and redepositing the dissolved portions within thegaps of the polycrystalline structure.

Many variations are possible from the foregoing general embodimentwithout departing from the scopeof the present invention. Following arethree specific embodiments which will serve as examples in furtherdescribing the aspects of the present invention.

In a preferred embodiment, first a mixture of an alcohol, water and aparticulate pyroelectric material is prepared. The mixture is then ballmilled until the pyroelectric material is submicron size and arelatively staand the liquid evaporated. As the alcohol evaporates, aportion of the pyroelectric material settles onto the substrate while apercentage of water increases in the remaining liquid. Thereafter, thewater also is evaporated leaving a thin film of the pyroelectricmaterial on the substrate.

Although any pyroelectric material may be used with the preceedingembodiment, preferably, triglycine sulfate (TGS), triglycine selenate(TGSe), triglycine fluoberyllate (TGFB) or combinations of these oranalogous pyroelectric compounds are utilized. Similarly, severaldifferent alcohols including isopropyl alcohol, methyl alcohol or highermolecular weight alcohols such as isobutyl alcohol may be used. Typicalmixtures which have proven effective are given in the following table.

TGS Films TGSe Films lsopropyl alcohol 20cc 20cc Water 20mg 20mgProelectric 131mg 144mg A suitable TGS is manufactured by EastmanOrganic Chemicals and is identified as Item No. 9921, Glycine Sulfate. Asuitable TGSe is manufactured by BDH Chemicals, Ltd. and is identifiedas Code No. 47060, Optran Triglycine Selenate.

The thickness of the deposited film, when uniformly deposited at bulkdensity, theoretically would be about IO Lm if the substrate werecovered with a 100 ml depth of one of the above mixtures. However, itwas found experimentally that a mixture depth of 25 mils produced at pmfilm of TGS thus indicating that the deposited film was about one fourththe bulk density of TGS.

Since it is desirable to have film density approach bulk density,thereby increasing the strength of the intergrain bonds and increasingthe adherence of the film on the substrate, an additional densificationstep may be used to further concentrate the film. Such densification isnot only important for added mechanical strength, but also results infilms that exhibit properties more nearly equal to that of bulk. Aneffective densification procedure is to treat the dry film withsaturated water vapor at a temperature approaching the boiling point ofwater. The water vapor dissolves a portion of the surface of the filmand permeates the porous film. Next, the film is dried, whereupon, thedissolved portion of the pyroelectric material is redeposited within theinternal gaps in the porous film. The densification procedure may berepeated several times if necessary, however, a significantdensification occurs on the first treatment. For example, it has beenfound experimentally that one treatment with water vapor can increasethe density ofa deposited TGS film by a factor of about 2.5 (e.g., to adensity of 80 percent bulk). Because of this densification step, anincrease in output voltage by a factor of about 100 was noted when thefilm is used as an infrared detector.

In another embodiment of the invention, the pyroelectric material is dryball milled before it is mixed with an alcohol and water. After thematerial is dry ball milled to submicron size, just enough alcohol andwater are added to the material to form a paste. The paste is thenapplied directly to the substrate, such as by use of a doctor blade, andthe alcohol and water are evaporated to form a pyroelectric film on thesubstrate. The film can then be densified as previously discussed.

This embodiment is particularly suitable for use with TGSe because ofthe difficulty that may be encountered with keeping TGSe in a stablesuspension. It should be noted, however, that the second embodiment mayresult in a film having greater variation in thickness and texture thanis obtainable with the preferred embodiment. The resultant films, asprepared by the second embodiment, are adequate for some functions butmay not possess the uniformity desired for other functions.

In a third specific embodiment, the pyroelectric material is mixed witha small amount of liquid during ball milling and then diluted to thedesired concentration following milling. This embodiment has theadvantage of increasing the yield of suspension from the rather timeconsuming milling procedure. After dilution, the same application,evaporation, and densification steps are carried out as in the preferredembodiment.

In any one of the preceding three embodiments, the application,evaporating and densifying steps may be repeated sequentially one ormore times in order to obtain a desired thickness of film.Experimentation has shown that such multi-layer build-up of the filmproduces relatively defect free layers since any defects, such as holesand cracks, are repaired by the deposition of subsequent layers.

The film deposited by any one of the foregoing embodiments can bedelinated into individual detectors by masking with some appropriatemeans (e.g., photoresist) whereafter the unwanted portions are etchedaway. A suitable etchant for triglycine sulfate is manufactured byTranscene Co., Inc. under the brand name Buffer HF.

1 claim:

1. A process for depositing a polycrystalline film on a substrate whichcomprises:

preparing a mixture of two volatile solvent liquids having a particulatepyroelectric material suspended therein, one of said volatile solventliquids having higher volatility and in which said particulate materialhas lower solubility than the other liquid;

applying said mixture to said substrate; and

evaporating said liquids to form a film of said material on saidsubstrate.

2. The process as defined in claim 1, including densifying said filmfollowing evaporation of said liquids.

3. The process as defined in claim 2, wherein said densifying comprises,treating said film with a vaporized solvent for said material topermeate said film, and drying said permeated film.

4. The process as defined in claim 3, wherein said vaporized solvent issaturated water vapor at a temperature approaching the boiling point ofwater.

5. The process as defined in claim 3, including repeating said applying,evaporating, treating and drying steps at least once.

6. The process as defined in claim 1, wherein said preparing a mixtureincludes, milling said mixture to reduce said particulate material tosubmicron size.

7. The process as defined in claim 6, wherein said milling is performedby ball milling said mixture.

8. The process as defined in claim 1, wherein said preparing a mixtureincludes, dry milling said particulate material to submicron size priorto addition of said liquids.

9. The process as defined in claim 8, wherein said dry milling isperformed by dry ball milling said material.

10. The process as defined in claim 1, wherein said preparing a mixtureincludes,-forming a combination of said particulate material andportions of said volatile solvent liquids, milling said combination, andadding the remaining portions of said volatile solvent liquids to obtaina predetermined concentration of said particulate material in saidvolatile solvent liquids.

1 1. The process as defined in claim 1, wherein one of said volatilesolvent liquids is water.

12. The process as defined in claim 1, wherein one of said volatilesolvent liquids is an alcohol.

13. The process as defined in claim 1, wherein said pyroelectricmaterial is a material selected from the group consisting of triglycinesulfate, triglycine selenate, triglycine fluoberyllate and combinationsthereof.

14. A process for depositing a polycrystalline film on a substrate whichcomprises:

preparing a mixture of water and an alcohol having a particulatepyroelectric material suspended therein;

applying said mixture to said substrate;

evaporating said alcohol from said mixture thereby concentrating saidpyroelectric material in said water and forming a film of pyroelectricmaterial on said substrate;

evaporating said water to deposit portions of said pyroelectric materialwithin said film;

treating said film of pyroelectric material with saturated water vaporat a temperature approaching the boiling point of water; and

drying said permeated film of pyroelectric material.

15. The process as defined in claim 14, including ball milling saidmixture to reduce said pyroelectric material to submicron size.

16. The process as defined in claim 14, including repeating saidapplying, evaporating said alcohol, evaporating said water, treating anddrying steps at least once.

17. The process as defined in claim 14, wherein said pyroelectricmaterial is a material selected from the group consisting of triglycinesulfate, triglycine selenate, triglycine fluoberyllate and combinationsthereof. =l

2. The process as defined in claim 1, including densifying said filmfollowing evaporation of said liquids.
 3. The process as defined inclaim 2, wherein said densifying comprises, treating said film with avaporized solvent for said material to permeate said film, and dryingsaid permeated film.
 4. The process as defined in claim 3, wherein saidvaporized solvent is saturated water vapor at a temperature approachingthe boiling point of water.
 5. The process as defined in claim 3,including repeating said applying, evaporating, treating and dryingsteps at least once.
 6. The process as defined in claim 1, wherein saidpreparing a mixture includes, milling said mixture to reduce saidparticulate material to submicron size.
 7. The process as defined inclaim 6, wherein said milling is performed by ball milling said mixture.8. The process as defined in claim 1, wherein said preparing a mixtureincludes, dry milling said particulate material to submicron size priorto addition of said liquids.
 9. The process as defined in claim 8,wherein said dry milling is performed by dry ball milling said material.10. The process as defined in claim 1, wherein said preparing a mixtureincludes, forming a combination of said particulate material andportions of said volatile solvent liquids, milling said combination, andadding the remaining portions of said volatile solvent liquids to obtaina predetermined concentration of said particulate material in saidvolatile solvent liquids.
 11. The process as defined in claim 1, whereinone of said volatile solvent liquids is water.
 12. The process asdefined in claim 1, wherein one of said volatile solvent liquids is analcohol.
 13. The process as defined in claim 1, wherein saidpyroelectric material is a material selected from the group consistingof triglycine sulfate, triglycine selenate, triglycine fluoberyllate andcombinations thereof.
 14. A process for depositing a polycrystallinefilm on a substrate which comprises: preparing a mixture of water and analcohol having a particulate pyroelectric material suspended therein;aPplying said mixture to said substrate; evaporating said alcohol fromsaid mixture thereby concentrating said pyroelectric material in saidwater and forming a film of pyroelectric material on said substrate;evaporating said water to deposit portions of said pyroelectric materialwithin said film; treating said film of pyroelectric material withsaturated water vapor at a temperature approaching the boiling point ofwater; and drying said permeated film of pyroelectric material.
 15. Theprocess as defined in claim 14, including ball milling said mixture toreduce said pyroelectric material to submicron size.
 16. The process asdefined in claim 14, including repeating said applying, evaporating saidalcohol, evaporating said water, treating and drying steps at leastonce.
 17. The process as defined in claim 14, wherein said pyroelectricmaterial is a material selected from the group consisting of triglycinesulfate, triglycine selenate, triglycine fluoberyllate and combinationsthereof.